The present invention pertains to vehicle audio systems and particularly to a speaker excursion control circuit for such audio systems.
Automotive sound systems have typically employed a plurality of speakers mounted in the instrument panels, door panels, rear package ledge or in floor consoles which permit the use of relatively large speakers of generally conventional design to allow for full range of both frequency and sound level for the reproduction of audio information. Although these speaker locations have provided adequate results, with the increased awareness of sound imaging through the increased popularity of, for example, home theater systems, purchasers of luxury vehicles expect and demand better audio performance from their vehicle's audio system. It has been discovered that by providing a new speaker platform, namely the headliner of the vehicle, and by using multiple, relatively small diameter speakers as opposed to the larger conventional speakers, the acoustical energy provided by the sound system can be precisely controlled in order to produce the desired "center-staging" and other acoustical effects for not only the driver of the vehicle but the remaining occupants as well. This desirable effect is achieved by providing relatively small (3"-5" diameter or other maximum dimension) speakers mounted directly on the vehicle headliner which is the decorative panel mounted to the roof of a vehicle. These speakers are employed for reproducing the more localizable frequencies of from about 150 Hz upwardly while conventional sub-woofers and woofers are employed and mounted in the vehicle for reproduction of the non-localizable lower frequencies. U.S. patent application Ser. No. 08/283,680; entitled Vehicle Audio System, filed on Aug. 1, 1994 discloses such an audio system for use in a vehicle.
One problem with the use of relatively small speakers in a vehicle is that they must be driven relatively hard (i.e. to their excursion limits) in order to generate sufficient acoustical energy. As a speaker is driven to its excursion limits, causing its voice coil to move to its outer limit of travel and leave the linear portion of the magnetic field of the permanent magnet, the diaphragm of the speaker becomes non-linear introducing audibly detectable and undesirable distortion. Some speakers are designed such that as the voice coil begins to leave the magnetic gap, a speaker suspension non-linearity is introduced to prevent the moving portions of a speaker from contacting its support frame thereby causing damage to the speaker. Although mechanically protecting the speaker, such a design also produces distortion in the reproduced audio information.
Therefore, it is desirable to provide, in connection with such speakers, a source of control signals which allow the speakers to be driven to, but not beyond, their excursion limits and to do so in an instantaneous fashion so that only those peak signals which might otherwise cause the speaker to introduce audible distortion are attenuated while at the same time simultaneously occurring signals which have little excursion requirement are passed. Thus, for example, the simultaneous occurrence of a trumpet tone and a drum beat which otherwise might cause distortion of the audible sound particularly affecting the trumpet tone, is desired to be controlled so that the drum beat only is briefly affected in order that the undistorted pure tone of the trumpet can be heard.
Although some systems have been proposed to prevent speaker damage through excessive motion, such circuits are designed only to operate on the overall energy applied to the speaker (i.e. the waveform envelope). As such, these systems do not differentiate between various signals with the result that both the peak signals capable of causing distortion and the simultaneously occurring high frequency signals are attenuated. Furthermore, these systems do not focus particular attention on the lower frequency signals which tend to require more speaker coil movement. Furthermore, the lower amplitude portions of the low-frequency signals are unnecessarily attenuated for a period of time after the peak, which might cause excess excursion, has passed.